CHICAGO—The jockeying to replace Jesse Jackson Jr. began before the ink was dry on the former congressman's resignation letter.

Among those expressing an interest: Chicago aldermen, a former NFL linebacker and a defense attorney who represented R&B singer R. Kelly and former Gov. Rod Blagojevich.

But as the field of would-be successors grows to a dozen or more names—one of whom may be another member of the Jackson family—party leaders and political analysts say a stampede of candidates could pose risks for the Democratic stronghold. Spread the field of candidates too thin, they say, and it becomes easier for a more conservative candidate—or anyone else party leaders don't want—to pull off a win.

The possibility so worried Democratic Congressman Bobby Rush, a close friend of the Jacksons, that within hours of the resignation he had these words for anyone thinking of running: "Cool your jets."

"My fear is that there is going to be so many wannabes blinded by ambition ... that we could find a tea party (candidate winning)," he said during a news conference. "That would be a travesty."

In a resignation letter sent Wednesday to House Speaker John Boehner, the 47-year-old son of a famed civil rights leader cited his ongoing treatment for bipolar disorder and admitted "my share of mistakes." He also confirmed publically for the first time that he is the subject of a federal probe and is cooperating with investigators.

Federal authorities are reportedly investigating Jackson's possible misuse of campaign funds. The House Ethics Committee is investigating his dealings with Blagojevich, who is serving a prison sentence for trying to sell President Barack Obama's former Senate seat.

Jackson has not been charged with wrongdoing. Attempts by The Associated Press to locate him for comment Thursday were unsuccessful, and his family also could not be reached.

Illinois Gov. Pat Quinn, a Democrat, is expected to schedule a primary and general election in the coming days.

Cook County Clerk David Orr said officials want to hold the elections on the same dates as previously scheduled elections for municipal officials, which are set for a Feb. 26 primary and an April 9 general election.

Orr said holding the 2nd Congressional District election those same dates would save money. But a federal judge will have to approve those dates because they do not conform to state law regarding deadlines for petition filing and when the election must be held.

Whenever it happens, the Democratic primary will be the race to win. The district, which stretches from Chicago's south side to several southern suburbs and rural areas, is heavily Democratic. Earlier this month voters there easily re-elected Jackson, who faced lesser-known Republican and independent candidates, despite the allegations swirling and the fact that he barely campaigned due to a leave of absence that began in June.

Longtime Chicago political strategist Thom Serafin said that makes Rush's doom-and-gloom prediction of a split field leading to a tea party victory highly unlikely.

Serafin said that because of the truncated election process, candidates who already are in office, are organized, and have shown they can raise money have the advantage. Those include longtime Chicago Alderman Anthony Beale, whose ward is in the district, and Alderman Will Burns, a former state representative.

"There's little doubt based on the district's history that a Democrat will be elected here," Serafin said. "The big question is who can organize and 'show me' they're the real deal? It's much simpler for someone who has been in the game for some time to put that together."

State Sen. Toi Hutchinson said she is weighing a bid. So is former U.S. Rep. Debbie Halvorson, who represented another district in Congress for one term before losing re-election in 2010. Halvorson was Jackson's first credible challenger in years when she ran a spirited campaign against him in the March primary.

"It looks like a lot of egos are jockeying for positions right now," Halvorson said. "I'm the only one who had the nerve to run against him. They need someone who is going to step in from day one."

Chicago attorney Sam Adam Jr. also has said he'd be interested. Adam is a recognizable face and name, having represented Blagojevich in his first trial in 2010, when the former governor was convicted on one count and the jury deadlocked on the remaining counts. Adam also was R. Kelly's lawyer when the singer was acquitted on child pornography charges.

Jackson's brother, Jonathan Jackson, and his wife Sandi Jackson—who currently serves as an alderman representing a part of the district—also have been mentioned, though neither has commented on a possible run.

Serafin said the Jackson name could draw a strong base of voters to the polls. If roughly 135,000 to 200,000 voters turn out and the field is split between multiple candidates, that base could be enough, he said.

Among the other possibilities are Cook County Board President Toni Preckwinkle and state senator-elect Napoleon Harris, who played in the NFL and owns two pizza franchises. His campaign aide, Curtis Thompson, told The Chicago Sun-Times Harris was taking the holidays to think about it.